For a number of years, aircraft manufacturers have desired to use the main electrical system of an aircraft for starting the main engines on the aircraft. One means by which this may be accomplished is to employ the dynamoelectric machine normally used as a generator and driven by an aircraft engine as a synchronous motor in an engine starting mode. Representative example of this approach may be found in commonly assigned U.S. Pat. No. 3,274,855 issued Sept. 27, 1966 to Reynolds et al; U.S. Pat. No. 3,786,696 issued Jan. 22, 1974 to Aleem; and U.S. Pat. No. 4,315,442 to Cordner.
Systems of the foregoing type as, for example, the Reynolds et al system, are acceptable for the purpose intended. However, the Reynolds system requires that hydraulic units employed as part of a constant speed drive therein be larger than would be necessary if the system were used solely for power generation.
The Reynolds et al system is schematically illustrated in FIG. 1 wherein there is shown an engine and associated gear box 10. An output shaft 12 from the engine and gear box 10 extends to a mechanical summer 14 which preferably will be a differential of known construction. The mechanical summer has three movable elements, one of which is connected to the shaft 12, and a second of which is connected via a shaft 16 to a dynamoelectric machine 18 which can alternatively serve as a generator during a power generating mode or as a synchronous motor during an engine starting mode.
The third rotary element of the differential 14 is connected via a shaft 20 to a normally fixed displacement hydraulic unit 22. Hydraulic lines 24 and 26 extend between the normally fixed hydraulic unit 22 and a variable displacement hydraulic unit 28. The variable hydraulic unit 28 is coupled via a shaft 30 to the shaft 16. Suitable gearing may be interposed between the various components and reference may be made to the previously identified Reynolds et al patent for further system detail.
In operating the system of FIG. 1 as a power generating system, torque from the engine and gear box 10 is transmitted to the dynamoelectric machine 18, now acting as a generator, via both the differential 14 and the shaft 16 on the one hand and the hydraulic units 22 and 28 together with the shafts 20 and 30 on the other. The torque necessary to operate the system in the power generating mode is at a particular level that is often less than the torque that must be transmitted in the opposite direction through the same components when the dynamoelectric machine 18 is acting as a synchronous motor for starting the engine 10. Consequently, in the Reynolds et al system, the hydraulic units 22 and 28 often must be made sufficiently large as to handle the higher torque level transmitted during a engine starting mode. This in turn means that these components will be physically larger than would be required if the system operated solely in a power generating mode.
The present invention is directed to overcoming one or more of the above problems.